Transparent conductive coatings are used in a wide range of applications such as displays (LCD, Plasma, touch screens, e-paper, etc), lighting devices (electroluminescence, OLED) and solar cells. The markets of these applications are moving towards flexible and printable products (“plastic electronics”), for which the current technology based on transparent conductive oxide (TCO) has numerous disadvantages having to do with, e.g., complexity of the manufacturing process, high cost, abundance of precursors, and relatively low conductivity. Consequently, much effort is devoted to finding alternatives for the most widely used tin doped indium oxide, ITO, which would provide high conductivity and yet high transparency.
Alternatives to obtaining transparent conductive coatings have been disclosed. Wu et al [1] demonstrated the application of carbon nanotubes as transparent electrodes, exhibiting transmittance properties in the IR range that are superior to ITO. Jiang et al [2] employed Al-doped ZnO films for OLED devices, and Wang et al [3] used ultra thin graphene films for solar cells.
Another alternative consists of a grid pattern, such as silver wire grids. However, even such an arrangement has several drawbacks, such as limitations in the printing process (resolution, substrate thickness). Garbar et al disclose formation of a transparent conductive coating by the use of emulations [4].
Deegan et al found that once a millimeter-size droplet of liquid containing solid particles is pinned to a substrate, upon drying of the droplet, the solid particles assemble into a ring [5]. Hu and Larson demonstrated that in the case of a mixture of liquids the Marangoni affect is also very significant [6]. Sommer [7] suggested a model for analyzing the five forces that affect the particles within the droplets and concluded that the main forces responsible for the ring formation are the interactions between the particles and the substrate and the flux that takes the particles to the periphery. Perelaer at al [8] and Kamyshny et al [9] showed that micrometric individual rings could be obtained by inkjet printing of dispersions of silica particles or microemulsion droplets.
It should be emphasized that in industrial inkjet printing usually a uniform pattern is required, and the “coffee ring effect” is an undesirable phenomenon[11,12].
It was previously reported that the deposition of one ring on top of another lead to the destruction of the first ring due to its re-dispersion [5]. Perelear et al utilized the inkjet printing method within the same printing parameters and obtained uniform size of droplets and consequently uniform rings [10]. Magdassi et al have disclosed [13,14] that in the case of dispersions of silver nanoparticles, this effect can lead to the formation of conductive layers of millimeter size rings without the need for sintering at high temperatures due to spontaneous close packing of the silver nanoparticles at the rim of the ring.